This invention concerns a glassy luminescent material which has a high luminous efficiency when excited either by short wavelength UV radiation (as emitted by a low pressure mercury vapor discharge), or by the long UV emission components of a medium or high pressure mercury vapor discharge lamp.
In fluorescent lamps the UV components of the gas discharge are currently converted into visible light in a thin layer of luminescent powders (phosphors) coating the inside surface of the lamp envelope. This layer of fluorescent powder could be eliminated by the use of a luminescent glass-envelope that is efficiently excited by the UV components of the gas discharge in the lamp.
In addition to the simplification in manufacturing that ensues from the use of a luminescent glass as lamp envelope, there is anticipated a reduced level of degradation of the active material responsible for the luminescent process, because of reduced exposure to the energetic plasma of the lamp discharge, and reduced process losses as currently arising from baking of the phosphor coating.
Previous studies of amorphous luminescent materials have concentrated on the following types of glasses: fused silica; simple alkali and alkaline earth silicates, borates and phosphates; and complex silicates. Although fused silica is a reasonable host for many luminescent ions, it has several disadvantages: (a) it can only accommodate small amounts of rare earth dopants (about 500 ppm); (b) even small (about 1 weight percent) additions of transition metal ions such as Mn.sup.2+ will destabilize fused silica by inducing glass-in-glass immiscibility or phase separation; and (c) fused silica requires extremely high temperatures for envelope fabrication.
Simple amorphous hosts for luminescent ions can be formed by the addition of alkali or alkaline-earth oxides to ternary systems of SiO.sub.2, B.sub.2 O.sub.3 and P.sub.2 O.sub.5 (B.sub.2 O.sub.3 and P.sub.2 O.sub.5 have not been studied individually to any great extent because of their extreme hygroscopicity). Although these glasses have high solubility for luminescent ions and relatively low melting temperatures, they are not commercially useful because of poor chemical resistance or instability with regard to phase separation.
In order to improve the chemical and phase stability, a variety of modifying ions must be added to the simple glass host. Although these ions can stabilize the glass, they also have the adverse effect of greatly increasing the absorption of short UV radiation by the host, resulting in poorer efficiency in producing visible emission.